Optical films, which are enjoying an increasing demand in the field of optoelectronics, are required to exhibit both birefringence and extremely high transparency. Use of polyethylene terephthalate (PET) films for this purpose has been studied.
PET films, which have only a small birefringence in the unstretched state, show significant birefringence when stretched uniaxially. However, it is difficult to control the birefringence due to crystallinity in the stretched film. Further, the domains in which crystallization has been produced during stretching cause light scattering, thus reducing transparency. Furthermore, because the glass transition temperature of the stretched PET film may be as low as 70.degree. C., the birefringence undergoes a gradual change during long term use.